The present invention generally relates to the field of electro-acoustical devices, and more particularly, is directed to an apparatus and method for producing high fidelity sound.
Electro-acoustical devices, or loudspeakers, emit variable acoustic pressures as a function of variations in the electrical signals that are fed into them. Such devices are formed of two main parts. The first is a transducer having electrical input terminals for the feeding of electrical signals and a movable output component whose displacements are linked to variations in the electrical signals. The second part is a movable component in the shape of a plate or diaphragm connected mechanically to the movable transducer component which ensures the emission of acoustic radiation corresponding to the electrical signals fed into the device.
Devices of this type have been widely distributed but normally exhibit a serious problem arising from the output obtained from the conversion of the energy applied to product displacement of the movable diaphragm. Indeed, the acoustic coupling associating the direct-projection diaphragm and the air produces a low-level output since the respective acoustic impedances of the diaphragm (high impedance) and the air (low impedance) are very different.
For this reason, loudspeakers have been built utilizing an acoustic impedance converter, i.e., a hollow funnel or horn in the general shape of a cone having a curved generator. This curve will, most advantageously, be exponential. The conversion of acoustic impedance takes place in the area located between the small section of the horn, or "throat," which corresponds to high impedance levels and where the movable diaphragm is installed, and the main portion of the horn, or "mouth," which corresponds to low impedance levels and which leads directly to the outside.
However, for a given mouth surface, the search for a high conversion ratio dictates a very small throat area. Consequently, this also leads to a small surface-area of the loudspeaker diaphragm, and thus, to severe limits to the power obtained from the loudspeaker.
To overcome these limits, it has been suggested that a cavity, or intermediate chamber, be placed between the movable diaphragm and the entrance to the throat. In this way, the surface-area of the diaphragm could be greater than that of the throat. Transmission of acoustical energy between the diaphragm and the throat would occur by means of the compression of air confined in the chamber.
This type of compression-chamber loudspeaker possesses, however, a serious disadvantage. The transmission of acoustical energy between the diaphragm and the throat, and therefore, the proper functioning of the loudspeaker, takes place only within a limited frequency range or band toward the treble frequencies. The compression chamber produces real attenuation in this zone.
Reducing the distance between the diaphragm and the throat permits raising of the cutoff frequency but introduces the risk of accidental contact of the diaphragm and the mouth during high-amplitude displacements in the bass range. Thus limiting, here again, the power that may be obtained from the loudspeaker. Furthermore, it should be noted that acoustical emission from this type of loudspeaker is severely directional, and occurs in the axis of the horn. In order to provide for a sizable angle of emission, typically on the order of 40.degree., multiple horns, juxtaposed one to the others, or multi-cellular horn, must be used.
FIG. 1 illustrates a horn-equipped loudspeaker according to conventional technology. It is comprised of an electro-acoustic transducer and a device for acoustic coupling with the air. The transducer contains, first, a coil of conducting wire 1 with two terminals 2 and 3. The coil is immersed in a magnetic field created by a magnet 4. The transducer contains, second, a movable diaphragm 5 to which the coil is attached. The air-coupling device contains a cavity or chamber 6 connected to a hollow horn 7 and having a small input opening 8, or "throat," and a large output opening 9, or "mouth."
A loudspeaker of this type operates in the following way. The feeding of A.C. electric voltage into the input terminal 2, 3 causes displacement of the moving coil 1 acted upon by the magnetic field created by the magnet 4, and, as a result, displacement of the transducer diaphragm 5. The acoustical energy thus created is propagated across the chamber 6 toward the output throat 8 of the horn from which it is finally emitted to the outside through the mouth 9.
The surface-area of the throat 8 must be small, to ensure effective coupling, i.e., adaptation of acoustic impedance, with the large surface-area of the mouth 9. However, the surface-area of the diaphragm 5 must be large, in order to produce a high level of acoustic power. These contradictory requirements are reconciled by the presence of a chamber or cavity 6, whose internal air volume transmits acoustical energy as a result of the elevated compressions and depressions which take place there, thus obtaining acoustical impedance adaptation between the diaphragm 5 and the throat 8.
However, the use of a compression chamber 6 constitutes a serious disadvantage, mentioned previously, consisting of the limitation of the transmission of treble frequencies and leading to an elevated "cutoff frequency" beyond which acoustical energy is no longer transmitted.